(a) Field of the Invention
The present invention relates to a plasma display panel, and particularly to a plasma display panel whose discharge efficiency is improved by employing a discharge gas having a high xenon (Xe) percentage, and the green brightness and lifespan characteristics are remarkably improved by increasing amounts of the green phosphor in proportion to the increase of the Xe percentage.
(b) Description of the Related Art
A plasma display panel (PDP) is a flat display device using a plasma phenomenon, which is also called a gas-discharge phenomenon since a discharge is generated in the panel when a potential greater than a certain level is applied to two electrodes separated from each other under a gas atmosphere in a non-vacuum state. Such gas-discharge phenomenon is applied to display an image in the plasma display panel.
A modern generally used plasma display panel is an alternating current (AC) driven plasma display panel, as shown in FIG. 1. The AC plasma display device has a fundamental structure in which front substrate 1 is disposed facing back substrate 3, with discharge space 5 between the two substrates. On front substrate 1, a pair of retaining electrodes (scanning electrode X, common electrode Y) is formed in a predetermined pattern, composed of transparent electrode 7 and metal film 9. Dielectric layer 11 is also coated thereon for the AC driving. The surface of dielectric layer 11 is coated with MgO passivation layer 13. On back substrate 3, address electrode A, dielectric layer 15, barrier rib 17, and phosphor layers 19R, 19G, 19B are formed.
The front substrate is disposed facing the back substrate and sealed. The internal space thereof is evacuated to reach a vacuum state, and the discharge gas is injected therein. The discharge gas may include any one or a mixture of inert gasses such as He, Ne, or Xe. The conventional discharge gas has 4–5% Xe content by weight, while it is currently being suggested to increase the Xe percentage in order to improve the photoemission efficiency. However, when the Xe percentage is excessively increased, it causes problems in that the lifespan of phosphors is decreased and the discharge voltage is increased.
Typically, the phosphor used for the PDP is a phosphor that is excited by ultraviolet rays. As the green phosphor has the highest percentage of white brightness among red (R), green (G), and blue (B) phosphors, the green brightness is the most important for improving the PDP brightness. Currently, Zn2SiO4:Mn and BaAl12O19:Mn are used for the green phosphor, and Zn2SiO4:Mn is the most popular due to its better brightness characteristics. However, it also is problematic in that the discharge characteristics are degenerated. The reason why the discharge characteristics of Zn2SiO4:Mn are degenerated will now be described in detail.
As shown in FIG. 1, since MgO layer 13 of front substrate 1 and phosphor layers 19R, 19G, 19B of back substrate 3 are directly exposed to the discharge space, the secondary electron emission coefficient of the MgO layer and the surface charge of the phosphor layer are directly affected by the amount of wall charge piled up on the phosphor layer and the MgO layer.
The phosphor layer has a different component composition depending on color. The surface electrification characteristics are also varied depending upon the kind of the material. During positive surface electrification, discharge failure rarely occurs, while during negative surface electrification, inferior discharge frequently occurs. This tendency is highly dependant on the driving system. In order to increase the discharge stability and to decrease the inferior discharge, it is preferable to select the R, G, B phosphor so that the surface electrification characteristic is positive regardless of the R, G, B color. Nevertheless, Zn2SiO4:Mn, the most popular green phosphor, has a negative surface electrification characteristic. Accordingly, when the PDP is driven by a driving waveform sensitive to the surface electrification characteristics of the phosphor layer, that is, the variation of the back substrate, the discharge voltage of the green cell becomes higher than those of the red cell and the blue cell.
The mechanism to increase the discharge voltage may be described as follows: upon the reset discharge, the characteristic of driving an alternating current plasma display during the real discharge, that is, before the discharge voltage is applied to the address electrode terminal, the wall charge is piled up. Before the discharge voltage is applied to the address electrode terminal, the wall charges having counter polarities are respectively piled up on the front substrate and the back substrate. Thereby, a voltage differentiation is generated between the front and back substrates.
Upon the voltage differentiation reaching a certain level, a voltage having the same polarity as the wall charge piled up on both the address electrode terminal and the scanning electrode terminal is applied to discharge. Thereby, the address discharge voltage is lowered by effectively piling the wall charge at an appropriate level. Before the discharge voltage is applied to the address electrode terminal, the cations pile up on the surface of the phosphor layer of the back substrate as a wall charge. As the Zn2SiO4:Mn having negative surface electrification characteristics is counterbalanced by the wall charge of cations, the green cell generates a smaller discharge voltage that those of the red cell and blue cell. Accordingly, the green cell of Zn2SiO4:Mn may require a higher address voltage compared to that of the red cell and the blue cell, and sometimes, discharge failure occurs.
In order to solve the problems relating to Zn2SiO4:Mn, Korean Patent Laid-Open Publication No. 2001-62387 discloses a green phosphor in which YBO3:Tb is added to Zn2SiO4:Mn. However, the obtained green phosphor has deteriorated color purity. Further, Korean Patent Laid-Open Publication No. 2000-60401 discloses a green phosphor in which a positive charged material of zinc oxide and magnesium oxide is added to Zn2SiO4:Mn. However, the green phosphor obtained from this method also causes problems in that the color purity and the lifespan are deteriorated. Further, Japanese Patent Laid-Open Publication No. 2003-7215 discloses that a mixture of manganese-activated aluminate green phosphor and terbium-activated phosphate or terbium-activated borate green phosphor can improve the driving voltage and the brightness failure. Nonetheless, it cannot improve the persistence of the green phosphor.
The above mentioned green phosphors exhibit a somewhat satisfactory brightness characteristic in the case of the discharge gas charged to the PDP being a 4–5% Xe percentage. On the other hand, in a high photoemission efficient PDP in which the Xe percentage is more than 6%, the phosphors do not exhibit a satisfactory brightness characteristic. Therefore, there is a need for a phosphor exhibiting good brightness and a good lifespan characteristic in discharge gas having a high Xe percentage.